1. Field of the Invention
The present invention relates to a refrigerating system of a vapor compression type, provided with a pressure control valve for controlling a pressure of a refrigerant at an output side of a heat emitter. The present invention is suitably used for a refrigerating system where a carbon dioxide (CO.sub.2), as a refrigerant, is used at a super critical region.
2. Description of Related Art
Recently, it has been required to avoid the use of FREON (chlorofluorocarbon) as a refrigerant in refrigerating systems. Thus, Japanese Unexamined Patent Publication No. 7-18602 discloses a vapor compression type refrigerating system where carbon dioxide (CO.sub.2) is used as a refrigerant in place of freon.
The carbon dioxide has a critical temperature as low as 31.degree. C., which makes it possible for the temperature of the carbon dioxide at the outlet of the heat emitter to be higher than the critical temperature, so that the carbon dioxide is prevented from being condensed even at the outlet of the heat emitter. In other words, an increased difference in a enthalpy by condensing the refrigerant can not be obtained as is the case in the conventional refrigerating system using FREON. Thus, in order to increase an enthalpy difference, an increase in the pressure of the carbon dioxide at the outlet of the compressor, i.e., the pressure at the inlet of the heat emitter, is essential. However, an increase in the outlet pressure of the compressor may cause a coefficient of performance to be worsened.